Nanomaterials have been used in a wide range of fields such as semiconductors, biotechnology, and catalysts. In particular, metal nanoparticles have been applied to various usages. Metal particles having a particle diameter on the nanometer order (particle diameter of less than 10 nm) are called metal nanoparticles, and as compared with bulk substances, in such metal nanoparticles, surface sites of a corner, an edge, and a terrace having high coordinative nonsaturability increase, so that the proportion of atoms present on the surface increases, and thus high activity in the reaction process is exhibited in many cases. For example, as one of use applications, it is known that a catalyst in which gold nanoparticles are fixed on the surface of a titanium oxide exhibits high CO oxidation characteristics at normal temperature. Therefore, there is a demand for a method for obtaining metal nanoparticles without impairing characteristics required according to use application.
Herein, there are generally two types of method for manufacturing metal nanoparticles: a breakdown method based on a physical technique and a buildup method based on a chemical technique. Of these, the buildup method does not require as large dedicated machines as those for the breakdown method, and accordingly, it is widely employed. As the buildup method, a method is known in which metal ions are chemically reduced in a solvent.
As the buildup method, for example, numerous methods for synthesizing silver nanoparticles in an aqueous solution have been reviewed, and typically, the Carey Lea method has been reviewed in which an aqueous solution of silver nitrate is added into an aqueous solution of a ferrous salt and a citric acid salt. In these methods, a dispersion liquid containing silver nanoparticles having a particle diameter on the order of 10 nm can be obtained. This dispersion liquid is excellent in high dispersion stability and narrow particle size distribution. Further, it is known that properties of metal nanoparticle dispersion liquids such as a silver nanoparticle dispersion liquid are greatly changed by controlling the particle diameters, particle size distribution, shapes, and the like of the metal nanoparticles.
One example of the method for manufacturing such metal nanoparticles includes a manufacturing method in which, in order to control the shapes and particle diameters of silver nanoparticles (silver powder), a slurry containing an amine complex of a silver salt and an amine complex of a heavy metal salt acting as a habit modifier during a reduction reaction, is mixed at once with a solution containing potassium sulfite used as a reducing agent and a gelatin used as a protective colloid, the amine complex of the silver salt is reduced, and silver nanoparticles thus generated are collected. (For example, see Patent Document 1.)
As a simple method of supporting gold nanoparticles on the surface of a basic or amphoteric metal oxide such as a titanium oxide, a deposition precipitation method is known. However, in the deposition precipitation method, if an oxide is not an oxide in which an isoelectric point is pH of about 5 or more, the gold nanoparticles cannot be supported. Therefore, an oxide support not corresponding to the above-described oxide, such as silica, zeolite, or clay, or a non-oxide support such as activated carbon or a porous resin cannot support the gold nanoparticles, and a problem arises in that there is limitation on use application according to selection of a constituent material for a support (for example, see Non-Patent Document 1).
Further, as a method of supporting gold nanoparticles on the surface of a support, a solid phase mixing method and a grafting method are exemplified. In these methods, the gold nanoparticles can be supported on the surface of activated carbon. However, in these methods, an expensive reagent such as gold acetylacetonato and expensive equipment such as a vacuum apparatus are necessary so that a problem arises in that the manufacturing cost increases.
Further, as the method of supporting gold nanoparticles on the surface of a support, a colloidal fixation method is exemplified. In the colloidal fixation method, there is no limitation on a support material as described above, and the metal nanoparticles can also be supported on the activated carbon or the like. In the colloidal fixation method, in order to prevent aggregation of gold nanoparticles in colloid, or the like, it is necessary to protect the gold nanoparticles with a protecting agent such as polyvinylpyrrolidone (PVP), support the gold nanoparticles on a support, and then remove the protecting agent by washing and calcination. However, in a case where a support is a material that does not endure a high temperature (about 200° C. to 300° C.), such as activated carbon, since the protecting agent is removed only by washing, the protecting agent adhering to the surfaces of the gold nanoparticles cannot be sufficiently removed, and a problem arises in that activity of the gold nanoparticles as a catalyst deteriorates.
As an example of the method for synthesizing a metal nanoparticle dispersion liquid, in order to suppress the self-assembly and to speed up the reduction reaction, a method for synthesizing a gold nanoparticle dispersion liquid has been proposed in which a chloroauric acid (HAuCl4) solution is mixed with a reducing agent (sodium borohydride, citric acid, ascorbic acid, or the like) by using a microreactor (micromixer) so that gold ions are reduced to generate gold atoms (for example, see Non-Patent Document 2). The microreactor used in the synthesis method is configured such that liquids pass through multiple tubular flow paths and then merge to mix with one another, and such a microreactor makes it possible to decrease the volume of a reaction solution to be mixed and to increase the mixing speed. For this reason, the microreactor enables highly efficient mixing and high-speed reduction reaction, and the self-assembly of gold atoms can be suppressed by a dispersant to be added to the microreactor.
However, the above-described one example of the synthesis method can only roughly control properties of metal nanoparticles, such as particle diameters, particle size distribution, shapes, and the like, by utilizing the properties of the chloroauric acid solution and the reducing agent, the structure of the microreactor, and the like, and thus, it is difficult to precisely control the properties of metal nanoparticles. For this reason, it is difficult to obtain metal nanoparticles having desired properties. Since the inner diameters of tubular flow paths of a microreactor are just about 100 μm in many cases, a reaction product adheres to inner walls of such tubular flow paths, and a problem arises in that the reaction product cannot be efficiently generated when the tubular flow paths are clogged and it is difficult to perform mass production.
Further, as another example of the method for synthesizing a metal nanoparticle dispersion liquid, in order to precisely control properties of metal nanoparticles and to efficiently generate the metal nanoparticles, a method for synthesizing a metal nanoparticle dispersion liquid has been proposed in which while a strong electric field is generated between two electrospray nozzles which are disposed to face each other in the air by applying positive and negative potentials, respectively, to the two electrospray nozzles, solutions of a metal salt and a reducing agent are respectively supplied to the two electrospray nozzles at constant flow rates, and droplets respectively charged to the positive and negative potentials are sprayed from the electrospray nozzles so that these droplets collide and mix with each other in the air by an electrostatic interaction (for example, see Patent Document 2).
Furthermore, the above-described other example of the synthesis method has such a problem that droplets charged to the positive and negative potentials do not efficiently collide with each other because the sprayed droplets diffuse in the air. Further, even when the droplets collide with each other, most of the reaction products made of the droplets by collision, either diffuse in the air or adhere to the wall surface. This results in a problem that the yield of the reaction product is low. Further, in a case where spraying is performed in a liquid, it is necessary to select a solvent that does not have an influence on the liquid-phase conductivity, and thus there is a limitation on the solvent to be used. Further, a problem arises in that the droplets enlarge as the sprayed amount is increased, and this results in an enlargement of sizes of metal nanoparticles.    Patent Document 1: Japanese Unexamined Patent Application, Publication No. H11-106806    Patent Document 2: PCT International Publication No. WO2012/173262    Non-Patent Document 1: Gold Nanotechnology: Fundamentals and Applications, Chapter 9, supervised by Masatake Haruta, CMC Publishing CO., LTD., p. 116-126 (2009)    Non-Patent Document 2: H. Tsunoyama, and two others, “Microfluidic Synthesis and Catalytic Application of PVP—Stabilized˜1 nm Gold Clusters,” Langmuir, (U.S.), 2008, Vol. 24, Issue number 20, p. 11327-p. 11330